In an experiment, DNA is allowed to replicate in an environment with all necessary enzymes, dATP, dCTP, dGTP, and radioactively labeled dTTP (³H thymidine)for several minutes and then switched to nonradioactive medium. It is then viewed by electron microscopy and autoradiography. The drawing below represents the results.
Grains represent radioactive material within the replicating eye.
Figure 16.2
Which is the most likely interpretation?
A)There are two replication forks going in opposite directions.
B)Thymidine is only being added where the DNA strands are furthest apart.
C)Thymidine is only added at the very beginning of replication.
D)Replication proceeds in one direction only.

Question

In an experiment, DNA is allowed to replicate in an environment with all necessary enzymes, dATP, dCTP, dGTP, and radioactively labeled dTTP (³H thymidine)for several minutes and then switched to nonradioactive medium. It is then viewed by electron microscopy and autoradiography. The drawing below represents the results. 
Grains represent radioactive material within the replicating eye.
Figure 16.2
Which is the most likely interpretation?
A)There are two replication forks going in opposite directions. 
B)Thymidine is only being added where the DNA strands are furthest apart. 
C)Thymidine is only added at the very beginning of replication. 
D)Replication proceeds in one direction only.

Quizplus · Accepted Answer

The presence of grains on both sides of the replication bubble and the fact that they are oriented in opposite directions suggests that there are two replication forks moving in opposite directions. This is a characteristic of bidirectional replication, which is the most common mode of DNA replication in cells. Thymidine is being incorporated throughout the replication process, not just at the beginning or where the strands are furthest apart. The presence of radioactively labeled material in both strands also indicates that replication is not proceeding in one direction only.

In an Experiment, DNA Is Allowed to Replicate in an Environment